In this application continued support is requested to study the connections and organization of the cerebral cortex of the rat and monkey. During the current grant period a combined Golgi and electron microscope technique was deveoped. This technique allows individual neurons to be first examined in the light microscpe, before they are gold toned and deimpregnated for additiona study in the electron microscope. This is the principal method to be used in the proposed research. Using the technique the fine structural characteristics of specific types of cortical neurons will be examined as well as the patterns of synapases on their perikarya and dendrites. The axons from these neurons will be followed to their terminals so that their synaptic junctions and neuronal elements postsynaptic to them can be identified. In this way it is intended to study the details of the intrinsic connections of neurons in the visual cortex. Thalamic afferents to layer IV of the rat and monkey visual cortex will also be examined by combining this Golgi/EM method with experimenal degeneration. This makes it possible to determine exactly which neurons receive the geniculocortical afferents. While these studies are progressing work will be commenced on the monkey auditory cortex, in which we will begin to examine both the callosal and thalamic afferents, to determine if they both synapse with the same types of neurons. The clusters of apical dendrites will also continue to be studied.